I figured out the problem afterwards. The tool’s “blend mode” setting was set to “add (glow)” so it would only go on a space with color on it. Which means if I want to do it on a separate layer or an area surrounding the space I need to set it to “normal” and the layer to “add (glow”). This is what I mean about playing with the settings.
This was very interesting and enlightening to see how you used technology in coloring the He-Man comic strip.